[TenTec] No SSB from Omni D

Tue, 7 Oct 2003 19:37:36 -0700 (PDT)

Hi Jay,

Well, I don't know what the bandwidth is of your
'scope. If is in excess of 10Mhz. (Mine is 5Mhz.) you
should be able to chect the output of the balanced
modulator at 9Mhz. You should see the classic pattern
shown in the ARRL hand book when you speak into the
microphone. You should see a similar pattern on the
output of the 9Mhz. Xtal filter.

I have not been able to locate my book, since I last
sent you e-mail. I have a number of people here using
this PC and a 16 month old who touches everything.

I performed my tests on 160Meters and viewed the
output after the MC1496 Mixer, but I would perform the
test I just noted about. You need to look at the 9Mhz.
output of the balance modulator and then at the output
of the Xtal filter. My belief is that you will find
the signal is significantly attenuated at the output
of the filter.

Do you have a frequency counter? This instrument
should be used to check the output of the carrier
oscillator. This Xtal oscillator is used for LSB, USB,
and CW. The difference is the amount of loading
capacitance that is placed in the circuit for each
mode. It does not take much of a frequency error with
the sharp Xtal filter to attenuate the signal
significantly. There is significant room for an error
in the 200A1 carrier frequency and still get
sufficient output from this circuit.

Let me know what the bandwidth is of your 'scope and
if you have access to a frequency counter.

Regards...

Ken

P.S. If you can scan the SSB generator board and send
me the image file. I can view it here and make an even
better diagnosis for you.

AE1X:kes

--- Jay Eimer <[email protected]> wrote:
> Okay, I borrowed a scope and checked some things. 
> One thing I confirmed is
> that my mic is getting to the SSB Gen board.  Turns
> out, out of 4 mics that
> I tried only one was working with one other
> marginal.  Once I tried with the
> good mic I got output enough that a friend was able
> to hear me on 28.330.
> Unfortunately, he was on his HTX-10 (radio shack)
> mobile sitting in my
> driveway.
> 
> He heard me up to about 2 miles away, then my signal
> faded.  I could hear
> him about twice that distance.  Note that he was on
> a base loaded vertical,
> and I was using an 132 ft. dipole and a tuner so the
> polarization was
> mismatched.
> 
> I tried making some contacts last night with the
> good mic, no go.  It seems
> that I'm getting out, but very, very weak (5W or
> less).
> 
> So I did some other checking with the 'scope. 
> Here's some of the anomalies
> I found.
> 
> I have modulation at the MIC test pin on the SSB Gen
> board.
> 
> I have modulation at pin 7 of the dual op amp IC1. 
> This is the output of
> the first stage amp IC1b.
> 
> I have an unmodulated 4V dc at pin 3 of IC1.  This
> doesn't make sense,
> especially since the manual lists a test voltage
> (SB-N receive with no
> drive) of 0V dc.
> 
> I have no output (unmodulated ~4V) out of pin 1
> (final output of the second
> stage op amp, IC1a) that feeds into IC2.
> 
> I'd suspect the IC, except for a further anomaly.  I
> appears that there is a
> voltage divider/step down circuit.  Out of pin 7 on
> IC1, it goes through R25
> and feeds pin 6 (feedback) and divides to R26.  This
> feeds one side, while
> Reg+ feeds the other side of voltage divider through
> R19 and R23.  R23 feeds
> R24 and then pin 3 (+ Wiper?) and R19 feeds R18 and
> then Pin 1 to Pin 1 of
> IC2.  Where this is wierd is that every resistor
> shows 4V (no drop) on both
> sides.  This doesn't seem right, but I'm no expert.
> 
> So, is this the same IC that you had a problem with?
>  Or do I have a bad
> component somewhere else on the board?  I suspected
> a bad cap was blocking
> modulation, but I can't see which one that would be,
> since the problem
> appears to be between output of pin 7 to input of
> pin 2 and or 3 of IC1.
> 
> What I think I need most is a test plan to isolate
> which of these might be
> the problem.
> 
> Thanks,
> 
> Jay
> AD5PE (just got my new call last Thursday!!!)
> 
> -----Original Message-----
> From: [email protected]
> [mailto:[email protected]]On Behalf Of
> Kenneth Stringham
> Sent: Tuesday, September 30, 2003 11:02 AM
> To: [email protected]
> Subject: Re: [TenTec] No SSB TX from Omni D
> 
> 
> Hi Jay,
> 
> My first question is: do you have the Operator's
> Manual. This includes a detailed description of the
> circuitry and the schematic. If you do not, I have
> one
> here that you can purchase, I sold my 546C recently
> because of my need for cash.
> 
> You problem will be located by looking at the SSB
> generator board. This radio generates the CW signal
> by
> moving the Carrier Oscillator 750Hz so it is inside
> the filter pass band and then it unbalances the IC
> balanced modulator. My guess is that you have
> problem
> with the circuitry that moves the oscillator
> frequency
> for LSB/USB. The frequency is offset to far down the
> filter pass band. One of mine had a defective
> balanced
> modulator IC which I replaced and all was well.
> 
> rest of original snipped....
> 
> 
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